1. Field of the Invention
The present invention relates to adjusting waveforms of optical filters used in Dense Wavelength Division Multiplexing (DWDM) systems, and more particularly to adjusting wavelengths of optical filters used in DWDM systems to obtain desired transmission and reflection.
2. The Prior Art
Demand for increased transmission capacity in fiber optical communications systems is unrelenting. Conventional transmission technology is increasingly unable to satisfy demands for higher transmission capacity and speed. Dense Wavelength Division Multiplexing (DWDM) technology has helped satisfy demand, and is now in widespread use in optical communications throughout the world. A DWDM system multiplexes a plurality of signals of different wavelengths into single optical fiber at an initial end of the optical fiber. The multiplexed signals are then demultiplexed into a plurality of different optical fibers at a terminal end. Each demultiplexed signal is then output to the end recipient. DW DM systems can increase transmission capacity by ten times or more. The heart of the transmission technology of DWDM systems is multiplexing many signals having different wavelengths into one fiber, and then demultiplexing the multiplexed signals into a plurality of different fibers. The device that achieves this function is a multiplexer/demultiplexer.
Nowadays there is a variety of multiplexer/demultiplexers in use, including multi-dielectric thin film filters, diffraction gratings, fiber bragg gratings (FBGs) and arrayed waveguide gratings (AWGs). Multi-dielectric thin film filters can achieve low insertion loss and high isolation of the multiplexer/demultiplexer. Multi-dielectric thin film filters also enjoy low production costs and established technology, and are therefore in widespread use.
FIGS. 1A, 1B, 2A and 2B show two conventional DWDMs using multi-dielectric thin film filters as the basic wave division device. Referring to FIGS. 1A and 1B, the wave division device comprises a biporose pigtail 51, a gradient index (GRIN) lens 52 and a filter 53 glued on one end of the GRIN lens 52. The biporose pigtail 51 is usually made from a glass rod or other suitable body, and typically has two holes defined therein. Multiplexed signals are transmitted to the GRIN lens 52 through an input fiber 54 inside the biporose pigtail 51. The GRIN lens 52 acts as a convergent lens by converting the multiplexed signals to parallel or near-parallel light, and then transmits the light to the input surface of the filter 53. The filter 53 is pre-formed such that it allows only one specific wavelength xcexm to be transmitted therethrough, and reflects all other wavelengths. The reflected signals are then converged by the GRIN lens 52 to enter a return optical fiber 55. The wavelength signal specific to the filter 53 is separated from the multiplexed signals, and transmitted to an output optical fiber (not shown). The input and return optical fibers 54, 55 are symmetrically disposed on opposite sides of a central longitudinal axis of the biporose pigtail 51. Therefore, the reflected signals from the GRIN lens 52 can be completely input into the return optical fiber 55.
Referring to FIGS. 2A and 2B, the wave division device comprises a biporose pigtail 61, a GRIN lens 62 and a filter 63 glued on one end of the GRIN lens 62. Input and return optical fibers 64, 65 are disposed together in a single hole defined in a central longitudinal axis of the biporose pigtail 61
In both these conventional DWDMs, a face of the GRIN lens 52, 62 that is joined to the filter 53, 63 is at a right angle to a central longitudinal axis of the GRIN lens 52, 62. 32-channel conventional DWDM systems are already in commercial use, and the center-wavelength bandwidth is now as small as 0.8 nm or even 0.4 nm. Therefore it is very important to accurately set the center-wavelength of a particular DWDM. Even a minute error in setting of the center-wavelength results in serious consequences such as channel cross talk and failure of transmission to the end recipient. With current technology, precisely setting a particular center-wavelength is very problematic. This is made all the worse because the face of the GRIN lens 52, 62 that is joined to the filter 53, 63 is at a right angle to the central longitudinal axis of the GRIN lens 52, 62; as a result the center-wavelength will be unable to be adjusted, and then the filter will be a no good (NG); unfortunately the case as described above often happens. So it greatly increases the production cost and reduces the efficiency of manufacture.
In addition, manufacturing error such as thickness of layers of the filter may cause the transmitting center wavelength thereof slightly incorrect; however, there is no means for balancing or compensating the incorrect of the transmitting center wavelength of the filter in the prior art.
To solve the problems of the prior art, the present invention provides a structure for adjusting waveforms of optical filters used in a DWDM system comprising a filter, a GRIN lens and a biporose pigtail with two holes therein. The holes are parallel to a center-axis of the pigtail but at different distances from the center-axis thereof, and an input and return optical fiber are secured within the two holes. The GRIN lens is provided with a first end coupled with the biporose pigtail; thus, signals from the input fiber can input the lens and the reflected signals from the lens can enter into the return fiber. The GRIN lens further defines a second end angularly to the axis thereof. The filter transmits a determined wavelength and is joined with the second end of the GRIN lens.
In addition, the present invention provides a method for adjusting waveforms of optical filters used in a DWDM system, comprising: measuring the actual center-wavelength of the filter; determining a difference between the actual center-wavelength and a desired center-wavelength of the filter; determining an angle xcex3 of the second end of the GRIN lens and distances r1, r2 of the two holes from the center optical axis of the pigtail that will yield the desired center-wavelength; grinding the second end of the GRIN lens to obtain the determined angle xcex3 and forming the pigtail to obtain the determined distances r1, r2; adhering the filter to the second end of the GRIN lens; and integrating the formed pigtail with the combination of the filter and the GRIN lens.
Accordingly, an object of the present invention is to provide a structure and method of adjusting waveforms of optical filters used in a DWDM system which decreases production costs and increases production yields.
Another object of the present invention is to provide a structure and method of adjusting waveforms of optical filters used in a DWDM system which freely balances the uncorrectable transmitting center wavelength of the filter.
Other objects, advantages and novel features of the present invention will be apparent from the following detailed description of preferred embodiments thereof with reference to the attached drawings, in which: